Spiral wound membrane module adapted for high recovery

ABSTRACT

A spiral wound membrane module adapted for hyperfiltration and including at least one membrane envelope and feed spacer sheet wound about a central permeate tube to form an inlet and outlet scroll face and an outer periphery,
         wherein the feed spacer sheet includes:   i) a feed entrance section extending along the permeate collection tube from the inlet scroll face toward the outlet scroll face,   ii) a feed exit section extending along the outer periphery from the outlet scroll face toward the inlet scroll face, and   iii) a central feed section located between the feed entrance section and the feed exit section; and   wherein the feed entrance section has a median resistance to flow in a direction parallel to the permeate collection tube that is less than 25% of the median resistance to flow of the central feed section in a direction perpendicular to the permeate collection tube.

FIELD

The invention generally relates spiral wound membrane module used inwater filtration.

INTRODUCTION

Spiral wound membrane modules are used in a variety of reverse osmosisand nanofiltration applications; see for example: U.S. Pat. Nos.5,458,774, 6,881,336, 8,337,698 and US 20040182774. Spiral wound modulesoperate in “cross-flow” mode with feed water passing across a membranesurface with a portion passing through the membrane as “permeate.” Thepercentage of feed solution passing through the membrane is referred toas the “recovery” or “recovery rate.” Depending upon the composition ofthe feed, operating at high recoveries can lead to scaling as salts andother dissolved solids in the feed become concentrated above theirsolubility limit. Spiral wound modules used in residential RO systemsare typically designed for recoveries between 20-35%. Operating athigher recoveries (e.g. above 35%) often leads to scaling as un-softenedresidential water sources can contain significant quantities of calciumand bicarbonate ions.

SUMMARY

The present invention is a spiral wound module designed to mitigatescaling. In a preferred embodiment, the module (2) includes at least onemembrane envelope (4) and feed spacer sheet (6) wound about a centralpermeate tube (8) to form an inlet (30) and outlet (32) scroll face andan outer periphery (38). The feed spacer sheet (6) includes:

-   -   i) a feed entrance section (50) extending along the permeate        collection tube (8) from the inlet scroll face (30) toward the        outlet scroll face (32),    -   ii) a feed exit section (52) extending along the outer periphery        (38) from the outlet scroll face (32) toward the inlet scroll        face (30), and    -   iii) a central feed section (54) located between the feed        entrance section (50) and the feed exit section (52).        The feed entrance section (50) of the feed spacer sheet (6) has        a median resistance to flow in a direction parallel to the        permeate collection tube (8) that is less than 25% of the median        resistance to flow of the central feed section (54) in a        direction perpendicular to the permeate collection tube (8).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective, partially cut-away view of a spiral woundmembrane module.

FIG. 2 is a perspective view of a partially assembled spiral woundmembrane module.

FIG. 3 is a perspective, partially cut-away view of an embodimentindividually showing the opposing scroll faces of a spiral woundmembrane module.

DETAILED DESCRIPTION

Reverse osmosis (RO) and nanofiltration (NF) are membrane-basedseparation processes where pressure is applied to a feed solution on oneside of a semi-permeable membrane. The applied pressure causes “solvent”(e.g. water) to pass through the membrane (i.e. forming a “permeate”)while “solutes” (e.g. salts) are unable to pass through the membrane andare concentrated in the remaining feed (i.e. forming a “concentrate”solution). Once concentrated beyond their solubility limit, retainedsalts (e.g. CaCO₃) begin to form scale on the membrane. Such scale isespecially problematic for long term operation of residential RO systemsat high recovery.

The present invention includes a spiral wound module suitable for use inreverse osmosis to (RO) and nanofiltration (NF) systems operating athigh recoveries. Such modules include one or more RO or NF membraneenvelops and feed spacer sheets wound about a permeate collection tube.RO membranes used to form envelops are relatively impermeable tovirtually all dissolved salts and typically reject more than about 95%of salts having monovalent ions such as sodium chloride. RO membranesalso typically reject more than about 95% of inorganic molecules as wellas organic molecules with molecular weights greater than approximately100 Daltons. NF membranes are more permeable than RO membranes andtypically reject less than about 95% of salts having monovalent ionswhile rejecting more than about 50% (and often more than 90%) of saltshaving divalent ions—depending upon the species of divalent ion. NFmembranes also typically reject particles in the nanometer range as wellas organic molecules having molecular weights greater than approximately200 to 500 Daltons. For purposes of this description, the term“hyperfiltration” encompasses both RO and NF.

A representative spiral wound membrane module is generally shown at 2 inFIG. 1. The module (2) is formed by concentrically winding one or moremembrane envelopes (4) and feed spacer sheet(s) (“feed spacers”) (6)about a permeate collection tube (8). Each membrane envelope (4)preferably comprises two substantially rectangular sections of membranesheet (10, 10′). Each section of membrane sheet (10, 10′) has a membraneor front side (34) and support or back side (36). The membrane envelope(4) is formed by overlaying membrane sheets (10, 10′) and aligning theiredges. In a preferred embodiment, the sections (10, 10′) of membranesheet surround a permeate spacer sheet (12). This sandwich-typestructure is secured together, e.g. by sealant (14), along three edges(16, 18, 20) to form an envelope (4) while a fourth edge, i.e. “proximaledge” (22) abuts the permeate collection tube (8) so that the insideportion of the envelope (4) (and optional permeate spacer (12)) is influid communication with a plurality of openings (24) extending alongthe length of the permeate collection tube (8). The active membraneregion (25) for each section of membrane sheet (10, 10′) corresponds tothe area of membrane through which liquid may pass into the envelope (4)during operation; (in contrast to non-active membrane regions (25′) thatare isolated by adhesives, tapes, etc. so that the flow of liquidthrough the membrane and into the inside of the permeate envelope isprevented). The module (2) may include a single envelope or a pluralityof membrane envelopes (4) each separated by a feed spacer sheet (6). Inthe illustrated embodiment, membrane envelopes (4) are formed by joiningthe back side (36) surfaces of adjacently positioned membrane leafpackets. A membrane leaf packet comprises a substantially rectangularmembrane sheet (10) folded upon itself to define two membrane “leaves”wherein the front sides (34) of each leaf are facing each other and thefold is axially aligned with the proximal edge (22) of the membraneenvelope (4), i.e. parallel with the permeate collection tube (8). Afeed spacer sheet (6) is shown located between facing front sides (34)of the folded membrane sheet (10). The feed spacer sheet (6) facilitatesflow of feed fluid through the module (2). While not shown, additionalintermediate layers may also be included in the assembly. Representativeexamples of membrane leaf packets and their to fabrication are furtherdescribed in U.S. Pat. No. 7,875,177 to Haynes et al.

During module fabrication, permeate spacer sheets (12) may be attachedabout the circumference of the permeate collection tube (8) withmembrane leaf packets interleaved therebetween. The back sides (36) ofadjacently positioned membrane leaves (10, 10′) are sealed aboutportions of their periphery (16, 18, 20) to enclose the permeate spacersheet (12) and to form a membrane envelope (4). Suitable techniques forattaching the permeate spacer sheet to the permeate collection tube aredescribed in U.S. Pat. No. 5,538,642 to Solie. The membrane envelope(s)(4) and feed spacer(s) (6) are wound or “rolled” concentrically aboutthe permeate collection tube (8) to form two opposing scroll faces(inlet scroll face (30) and outlet scroll face (32)) and an outerperiphery (38). The resulting spiral bundle is held in place by tape orother means. The scroll faces (30,32) of the module may then be trimmedand a sealant may optionally be applied at the junction between thescroll face (30, 32) and permeate collection tube (8) as described inU.S. Pat. No. 7,951,295 to Larson et al. An impermeable layer such astape may be wound about the circumference of the wound module asdescribed in U.S. Pat. No. 8,142,588 to McCollam. In alternativeembodiments, a porous tape or fiberglass coating may be applied to themodule's periphery.

Arrows shown in FIG. 1 illustrate general flow directions (26, 28) offeed and permeate through the module (2). More specifically, feed fluidenters the module (2) from the inlet scroll face (30) and exits themodule from the outlet scroll face (32). Permeate fluid flows along thepermeate spacer sheet (12) in a direction generally perpendicular to thepermeate collection tube (8) as indicated by arrow (28).

To better illustrate the feed flow pathway shown by dotted arrows (48),module (2) is shown in an unwound state including a membrane envelope(4) and feed spacer sheet (6) extending from permeate collection tube(8). The feed spacer sheet (6) preferably comprises a sheet of polymericweb or net material including a plurality of crossing filaments, similarto those available under the trade name VEXAR™ from Conwed Plastics oras described in U.S. Pat. No. 6,881,336 to Johnson. More specifically,the feed spacer sheet (6) includes:

i) a feed entrance section (50) extending along the permeate collectiontube (8) from the inlet scroll face (30) toward the outlet scroll face(32),

ii) a feed exit section (52) extending along the outer periphery (38)(i.e. adjacent the distal edge of membrane envelope (20)) from theoutlet scroll face (32) toward the inlet scroll face (30), and

iii) a central feed section (54) located between the feed entrancesection (50) and the feed exit section (52).

In a preferred embodiment, the feed entrance section (50) and centralfeed section (54) of the feed spacer sheet (6) each have a distinctmedian resistance to flow; wherein the term “resistance to flow” refersto the pressure drop per unit of distance at a water velocity of 1cm/second at 25° C. More specifically, the feed entrance section (50)has a median resistance to fluid in a direction to parallel to thepermeate collection tube (8) that is less than 25% of the medianresistance to flow of the central feed section (54) in a directionperpendicular to the permeate collection tube (8). In another preferredembodiment, the feed exit section (52) of the feed spacer sheet (6) alsohas a median resistance to flow that is less than 25% of the medianresistance to flow of the central feed section (54) in a directionperpendicular to the permeate collection tube (8). In this way, the feedentrance (50) and exit (52) sections effectively serve as low resistanceflow distributors for feed fluid to flow to and from the central feedsection (54). Preferably, the median resistance to flow perpendicular tothe permeate collection tube (8) in the central feed section (54) isgreater than 0.5 psi/ft, more preferably greater than 1 psi/ft, or evengreater than 2 psi/ft, when measured at 25° C. with an average flowvelocity of 15 cm/sec. The median resistance to flow parallel to thepermeate collection tube (8) in the feed entrance section (50) and/orfeed exit section (52) is preferably less than 1.0 psi/ft, morepreferably less than 0.5 psi/ft, or even less than 025 psi/ft, whenmeasured at 25° C. with a flow velocity of 15 cm/sec.

The feed spacer sheet (6) may be in the form of a single sheet withdistinct sections (feed entrance (50), feed exit (52) and central feed(54)) having different resistances to flow, or may comprise separatesections that may be optionally secured together to facilitate moduleassembly. For example, the feed spacer sheet (6) may be produced withsections having different thicknesses, free volume, number of filaments,angles between filament, and strand thinning Orientation of the feedspacer relative to the direction of flow (48) can also be used to varyflow resistance in a specified direction. For example, the same spacermaterial may be used within the central feed section (54) as in the feedentrance section (50) and feed exit sections (52) but can be made“distinct” by orientating individual filaments (e.g. at 90°) in a mannerto change its resistance to flow in a direction parallel to permeatecollection tube (8), (i.e. axis X). Preferably, the central feed section(54) contains a net oriented to provide lower flow resistance in thedirection perpendicular to the permeate tube (8). Preferably, the feedentrance section (50) and/or feed exit section (52) contains a netoriented to provide lower flow resistance in the direction parallel tothe permeate collection tube (8).

In another embodiment, resistance to feed flow parallel to the permeatecollection tube (8) may be reduced by modifying a component of the feedspacer sheet (6) in one or more sections throughout the feed spacersheet (6). For instance, regions of a net in the feed entrance section(50) and/or feed exit section (52) may be cut out. Preferably, removedsections are elongated and oriented in the direction of the permeatecollection tube (8). Alternatively, flow channels may be embossed into anet to make flow easier in the direction of the permeate tube (8). Inyet another alternative embodiment, the entire of spacer sheet (6) mayinclude a first spacer sheet type, and a lower resistance layer may beadded to overlap the first spacer sheet type in one or both the feedentrance and feed exit sections (50, 52) of the feed spacer sheet (6),thus lowering the resistance to flow within a given section. Moregenerally, the module (2) may include a first spacer sheet type locatedwithin the central feed section (52) and either the feed entrancesection (50) or the feed exit section (52) of the to feed spacer sheet(6) may include both a first spacer sheet type and an overlapping secondspacer type with the second spacer sheet type preferably having lessmedian resistance to flow in a direction parallel to the permeatecollection tube (8) than the first spacer sheet type. More preferably,the second spacer sheet type is a net oriented to have less resistanceto flow in the direction parallel to the permeate collection tube (8)than in the direction perpendicular to the permeate collection tube (8).The second spacer type may be affixed to the first spacer sheet type toaid in module rolling. The feed entrance section (50) and feed exitsection (52) of the feed spacer sheet (6) are shown in FIG. 2 as beingseparated from the central feed section (54) by dotted lines (56, 58).While not shown to scale in FIG. 2, the feed entrance section (50) andfeed exit section (52) each preferably comprise less than 20% (and morepreferably less than 15% or event 10%) of the total area of the feedspacer sheet (6) with the central feed section (54) comprising themajority (e.g. 60%, 75%, 90%, etc.) of the total area.

In the preferred embodiment shown, the feed entrance and exit sections(50, 52) are generally rectangular shaped and are located along thepermeate collection tube (8) and outer periphery (38), respectively. Ina yet another preferred embodiment, a majority (over 50% of area) of thefeed exit section (52) of the feed spacer sheet (6) is in planar contactwith the non-active membrane region (25′) of the membrane sheet (10),preferably at a location between the active membrane region (25) andmodule's outer periphery (38). In a still further preferred embodiment,the feed exit section (52) only contacts non-active membrane regions(25′) of a membrane sheet (10) at points distal to its active membraneregion (25).

In a preferred embodiment, feed flow into the module is restricted toareas concentrically located about the permeate collection tube (8) andspaced from the outer periphery (38). Similarly, feed flow out of themodule (2) is preferably restricted to areas adjacent to the outerperiphery (38) of the outlet scroll face (32). The means for restrictingflow into and out of the module are not particularly limited and includethe use of sealants (62) or tape (not shown) on the scroll faces (30,32). In an alternative embodiment shown in FIG. 3 optional cap members(33, 35) may be attached to respective inlet and exit scroll faces (30,32) of the module (2). Positioning means (37) on a cap member may alignthe cap member to the permeate tube (8) or outer periphery (38). Aninlet cap member (33) may restrict flow into the inlet scroll face (30)to a feed entrance region (60) near the permeate tube (8). An exit capmember (35) may restrict flow through the exit scroll face (32) to aregion near the outer periphery (38). Preferably, at least 50% and morepreferably at least 75% of feed liquid passing through the central feedsection (54) also passes through a feed exit region (64).

The embodiment shown in FIG. 3 also illustrates several other optionalfeatures. An impermeable layer (69), e.g. tape, is shown wrapped aboutthe peripheral surface (38). A brine seal (65) is located to thisimpermeable layer (69) and oriented to provide best sealing when highpressure is applied to the feed inlet region (60). On the opposite endof the module, corresponding to the feed exit region (64), an o-ring isshown as a sealing member (67) attached to the permeate collection tube(8), i.e. near the outlet scroll face (32). While not shown, thepermeate collection tube (8) may further to include a sealed end nearthe inlet scroll face (30).

In operation, feed flows into the feed entrance region (60) located onthe inlet scroll face (30) adjacent to the permeate collection tube (8),flows axially along the permeate collection tube (8) within the feedentrance section (50) and then flows radially through the central feedsection (54) toward the outer periphery (38) to the feed exit section(52) where feed subsequently flows axially to exit the module (2) at afeed exit region (64) located on the outlet scroll face (32) adjacent tothe outer periphery (38). Thus, in accordance with a preferredembodiment of the invention, feed flow encounters a relatively lowresistance to flow when entering the module and passing through the feedentrance section (50). This area of low resistance allows feed to beredirected in a radial direction while preventing “dead” regions nearthe permeate collection tube (8) where feed velocity may otherwise slow.Moreover, the feed exit section (52) allows feed flow to maintain highand uniform velocity across the active membrane (25) near the module'speriphery (38), where sealant concentration is highest. Because themodule's outer periphery (38) (i.e. distal end (20) of membrane envelope(4)), is where permeate back pressure is the greatest, flux is reducedat this location. As a consequence, scaling is much less likely tooccur, making the spiral wound membrane module capable of operating athigher recovery rates than conventional designs. Moreover, the subjectmodule is amenable to large scale manufacturing and may utilizeconventional infrastructure, pressure vessels, etc.

Materials for constructing various components of spiral wound modulesare well known in the art. Suitable sealants for sealing membraneenvelopes include urethanes, epoxies, silicones, acrylates, hot meltadhesives and UV curable adhesives. While less common, other sealingmeans may also be used such as application of heat, pressure, ultrasonicwelding and tape. Permeate collection tubes are typically made fromplastic materials such as acrylonitrile-butadiene-styrene, polyvinylchloride, polysulfone, poly (phenylene oxide), polystyrene,polypropylene, polyethylene or the like. Tricot polyester materials arecommonly used as permeate spacers. Additional permeate spacers aredescribed in U.S. Pat. No. 8,388,848.

The membrane sheet is not particularly limited and a wide variety ofmaterials may be used, e.g. cellulose acetate materials, polysulfone,polyether sulfone, polyamides, polysulfonamide, polyvinylidene fluoride,etc. A preferred membrane is a three layer composite comprising 1) abacking layer (back side) of a nonwoven backing web (e.g. a non-wovenfabric such as polyester fiber fabric available from Awa Paper Company),2) a middle layer comprising a porous support having a typical thicknessof about 25-125 μm and 3) a top discriminating layer (front side)comprising a thin film polyamide layer having a thickness typically lessthan about 1 micron, e.g. from 0.01 micron to 1 micron but more commonlyfrom about 0.01 to 0.1 μm. The backing layer is not particularly limitedbut preferably comprises a non-woven fabric or fibrous web mat includingfibers which may be orientated. Alternatively, a woven fabric such assail cloth may be used. Representative examples are described in U.S.Pat. Nos. 4,214,994; 4,795,559; 5,435,957; 5,919,026; 6,156,680; US to2008/0295951 and U.S. Pat. No. 7,048,855. The porous support istypically a polymeric material having pore sizes which are of sufficientsize to permit essentially unrestricted passage of permeate but notlarge enough so as to interfere with the bridging over of a thin filmpolyamide layer formed thereon. For example, the pore size of thesupport preferably ranges from about 0.001 to 0.5 μm. Non-limitingexamples of porous supports include those made of: polysulfone,polyether sulfone, polyimide, polyamide, polyetherimide,polyacrylonitrile, poly(methyl methacrylate), polyethylene,polypropylene, and various halogenated polymers such as polyvinylidenefluoride. The discriminating layer is preferably formed by aninterfacial polycondensation reaction between a polyfunctional aminemonomer and a polyfunctional acyl halide monomer upon the surface of themicroporous polymer layer. Due to its relative thinness, the polyamidelayer is often described in terms of its coating coverage or loadingupon the porous support, e.g. from about 2 to 5000 mg of polyamide persquare meter surface area of porous support and more preferably fromabout 50 to 500 mg/m².

The proto-typical membranes for reverse osmosis are FilmTecCorporation's FT-30™ type membranes, made by reaction of m-phenylenediamine and trimesoyl chloride. This and other interfacialpolycondensation reactions are described in several sources (e.g. U.S.Pat. Nos. 4,277,344 and 6,878,278). The polyamide membrane layer may beprepared by interfacially polymerizing a polyfunctional amine monomerwith a polyfunctional acyl halide monomer, (wherein each term isintended to refer both to the use of a single species or multiplespecies), on at least one surface of a porous support. As used herein,the term “polyamide” refers to a polymer in which amide linkages(—C(O)NH—) occur along the molecular chain. The polyfunctional amine andpolyfunctional acyl halide monomers are most commonly applied to theporous support by way of a coating step from solution, wherein thepolyfunctional amine monomer is typically coated from an aqueous-basedor polar solution and the polyfunctional acyl halide from anorganic-based or non-polar solution.

The pressure vessels used in the present invention are not particularlylimited but preferably include a solid structure capable of withstandingpressures associated with operating conditions. The vessel structurepreferably includes a chamber having an inner periphery corresponding tothat of the outer periphery of the spiral wound modules to be housedtherein. The pressure vessel may also include one or more caps or endplates that seal the chamber once loaded with one or more modules. Thevessel further includes a feed inlet located at one of end of thechamber, a concentrate outlet preferably located at the opposite end ofthe chamber, and at least one permeate outlet. The orientation of thepressure vessel is not particularly limited, e.g. both horizontal andvertical orientations may be used.

Examples of applicable pressure vessels, module arrangements and loadingare described in: U.S. Pat. Nos. 6,074,595, 6,165,303, 6,299,772 and US2008/0308504. Manufacturers of pressure vessels for large systemsinclude Pentair of Minneapolis Minn., Bekaert of Vista Calif. and BelComposite of Beer Sheva, Israel. The length of the chamber preferablycorresponds to the combined length of the modules to be sequentially(axially) loaded, e.g. 1 to 8 modules, see US 2007/0272628 to Mickols.The module of this invention is particularly useful as the last modulein a series of modules, so that it sees the highest feed waterconcentrations. In a preferred situation, water to be treated issupplied from upstream modules and enters the feed inlet section (50).Less than 50% of water to be treated is converted to permeate. Themajority water to be treated flows perpendicular to the permeate tubethrough the central feed section (54) and the feed exit section (52).Concentrated feed water leaving the module near the outer periphery (38)exits the vessel. A preferred embodiment comprises other modules withinthe vessel and the majority of other modules have a conventional design,wherein feed flow is predominantly oriented parallel to the permeatetube.

This invention is particular suitable to modules designed forresidential use, e.g. those have less than 2 m² and more preferably less1 m² of membrane area. A preferred length for such a module is less than0.5 m. A representative hyperfiltration module includes FilmTec's 1812configuration (e.g. TW30-1812), which is nominally 1.8 inches (4.6 cm)in diameter and nominally 12 inches (30 cm) long. This module can beequipped with the feed spacer sheet as described herein. Preferably,this inventive module would be the only module within a vessel. Thevessel containing an inventive module would include only one each ofconnections for feed, concentrate, and permeate streams.

Many embodiments of the invention have been described and in someinstances certain embodiments, selections, ranges, constituents, orother features have been characterized as being “preferred”. Suchdesignations of “preferred” features should in no way be interpreted asan essential or critical aspect of the invention. Expressed rangesspecifically include end points.

1. A spiral wound membrane module (2) comprising at least one membraneenvelope (4) and at least one feed spacer sheet (6) wound about acentral permeate tube (8) to form inlet (30) and outlet (32) scrollfaces and an outer periphery (36), wherein the feed spacer sheet (6)comprises: i) an inner section (50) extending along the permeatecollection tube (8) from the inlet scroll face (30) to the outlet scrollface (32), ii) an outer section (52) extending along the outer periphery(38) from the outlet scroll face (32) to the inlet scroll face (30),iii) a central section (54) located between the inner section (50) andthe outer section (52); wherein the inner section (50) of the feedspacer (6) encompasses an area that is less than 20% of the total areaof the feed spacer sheet (6); and wherein the inner section (50) of thefeed spacer sheet (6) has a median resistance to flow in a directionparallel to the permeate collection tube (8) that is less than 25% of amedian resistance to flow of the central section (54) in a directionperpendicular to the permeate collection tube (8) wherein the resistanceto flow is measured at a water velocity of 15 cm/second at 25° C.
 2. Thespiral wound module (2) of claim 1, wherein both the inner section (50)and central section (54) comprise a first spacer sheet type, and theinner section (50) of the feed spacer sheet (6) further comprises anoverlapping second spacer sheet type; wherein the second spacer sheettype has less median resistance to flow in a direction parallel to thepermeate collection tube (8) than the first spacer sheet type.
 3. Thespiral wound module (2) of claim 1, wherein the feed spacer sheet (6) isa net, both the inner section (50) and central section (54) comprise afirst spacer sheet type, and either bosses or apertures provided withinthe net in the inner section (50) of the feed spacer sheet (6) provideflow channels that reduce resistance to feed flow in the directionparallel to the permeate collection tube (8).
 4. The module (2) of claim1 further comprising a feed entrance region (60) located on the inletscroll face (30) adjacent to the permeate collection tube (8), a feedexit region (64) located on the outlet scroll face (32) adjacent to theouter periphery (38) and a feed flow pathway (48) extending from thefeed entrance region (60) to the feed exit region (64).
 5. The module(2) of claim 4 further comprising an exit cap member (35) that restrictsflow through the exit scroll face (32) to a region near the outerperiphery (38) such that at least 75% of feed liquid passing through thecentral feed section (54) also passes through the feed exit region (64).6. The module (2) of claim 1 wherein the feed exit section (52) of thefeed spacer sheet (6) has a median resistance to flow in a directionparallel to the permeate collection tube (8) that is less than 25% ofthe median resistance to flow of the central feed section (54) in adirection perpendicular to the permeate collection tube (8).
 7. Themodule (2) of claim 1 wherein: the membrane envelope (4) comprises amembrane sheet (10) comprising an active membrane region (25) throughwhich liquid may flow and a non-active membrane region (25′) from whichliquid flow is blocked, the feed exit section (52) of the feed spacersheet (6) is in planar contact with the membrane sheet (10), and whereinthe majority of planar contact between the feed exit section (52) andthe membrane sheet (10) is in the non-active membrane region (25′). 8.The module (2) of claim 2 wherein the second spacer sheet type is a netoriented to have less resistance to flow in the direction parallel tothe permeate collection tube (8) than in the direction perpendicular tothe permeate collection tube (8).
 9. The module (2) of claim 1 furthercomprising an impermeable layer (69) wrapped about the outer peripheral(38), and a brine seal (65) is located to the impermeable layer (69).